As one of the grandest castle complexes whose roots go back to the 12th century, Rabenstein Castle houses Europe's largest air to air heat pump in a historic building. The TOSHIBA VRF reference project demonstrates how an area of around 2,200 m2 can be heated economically and flexibly, while significantly reducing carbon emissions and conserving natural resources. Since the principle of the heat pump was discovered in 1856, a variety of different heat pump systems for different heat sources have been developed. In essence, air to air heat pumps obtain energy from the surrounding air, increase the temperature and use it for heating.